Police Scotland, in consultation with both clubs, made the ruling following trouble at Celtic's last away match against Motherwell earlier this month.
Firing smoke bombs and flares is a criminal offence at Scottish grounds but a number of pyrotechnics were let off in the away end at Fir Park that night while almost 175 seats were damaged during the match.
After the ban was imposed at McDiarmid Park a number of Celtic fans claimed they would take up the Perth club's offer of a refund on their ticket while others called for a total boycott of the match.
That preceded a statement from Celtic which claimed the ban had been overturned and that flags and banners would now be permitted in the ground.
But Police Scotland claimed they had no such discussion with Celtic, or St Johnstone, forcing Celtic to admit they had contacted officials at Inverness Caledonian Thistle, who the champions play on Sunday.
Celtic have refused to comment on the apparent gaffe but St Johnstone and Police Scotland say they will be releasing a statement this morning to clarify the situation.
Watch the action live from McDiarmid Park from 3.00pm on Boxing Day on Sky Sports 1